1. Field of the Invention
The invention relates to an electrophotographic image-forming method and an image-forming apparatus using the same.
2. Description of the Related Art
Electrophotographic methods have been commonly used in forming images in, for example, copying machines and laser beam printers. Developers known for use in electrophotographic methods include two-component developers containing toner and carrier particles and one-component developers containing only magnetic or nonmagnetic toner particles. The toner particles contained in such developers are usually produced by a kneading and pulverizing process.
In the kneading and pulverizing process, desired toner particles are produced by melt-kneading a resin such as a thermoplastic resin, a pigment, a charge control agent, and a releasing agent such as wax, cooling the resultant mixture, pulverizing the mixture into fine particles, and classifying the fine particles. In order to improve the flowability and the cleaning property of the toner, particles prepared in the kneading and pulverizing process are used as mother particles, and inorganic and/or organic fine particles are adhered to the surfaces of the mother particles, and the resulting particles are used as toner particles in some cases.
In the electrophotographic image-forming method, an electrostatic latent image formed on a latent image-holding member by an optical unit is developed with a developer in a developing step, giving a toner image. The toner image is transferred onto a recording medium such as recording paper in a transferring step, and the transferred toner image is finally fixed on the recording medium generally by application of heat and pressure in a fixing step.
Recently, electrophotographic technology has advanced rapidly from black and white technology to full-color technology. In a full-color electrophotographic imaging method, a full-color image is generally produced by using four color toners, one in black and three in three primary colors: yellow, magenta, and cyan.
Generally in the full-color electrophotographic imaging method, an original image is first subjected to color separation to obtain yellow image information, magenta image information, cyan image information, and black image information. An electrostatic latent image is formed on a photoconductive layer according to one image information, and the electrostatic latent image is then developed to give a toner image. The series of processes are repeated for the respective colors. The resultant toner images of different colors are aligned and superimposed on a recording medium so that the resulting full-color image coincides with the original image. The full-color image is then fixed on the recording medium.
The color toners used in the full-color electrophotographic imaging method should be mixed thoroughly in the fixing step. In this way, it is possible to improve color reproducibility and transparency of overhead projector (OHP) images and obtain full-color images having superior image quality. To improve color-mixing property, a low-molecular weight resin having a narrow melting temperature range is often used in the color toner.
To cope with the trend toward energy conservation, various toners having an improved low-temperature fixing property have been proposed (e.g., Japanese Patent Application Laid-Open (JP-A) Nos. 9-325520, and 8-234480, and Japanese Patent Application Publication (JP-B) No. 4-24702). In particular, toners having a so-called core-shell structure, which have a core layer and a shell layer coating the core layer, can be superior in low temperature-fixing property (e.g., JP-B No. 4-24702). These toners generally have a melting temperature range narrower than that of other conventional toners to improve the color-mixing property in a low temperature-fixing step.
On the other hand, in an image-forming apparatus implementing an electrophotographic method, input image information is converted to an input signal by, for example, a scanner, and the input signal is then subjected to various image processings such as filtering and pseudo-halftone processing in the image-processor, finally giving an output signal. Subsequently, an electrostatic latent image is formed according to the output signal.
When the input image information is a pictorial image such as photograph, a datum for one pixel is a multivalued datum. In contrast, in an image-forming apparatus forming an image on a recording medium such as recording paper, the number of gradations expressed per pixel is substantially much smaller.
To solve the nonconformance between the datum and the number of the gradations per pixel, such an image-forming apparatus employs a method of forming a so-called pseudo-halftone image. In the method, each pixel is divided into plural sub-pixels to improve resolution, and image density in each sub-pixel is expressed by an area modulation method. Pseudo-halftone processing is the image processing performed in the step of converting an input signal to a pseudo-halftone image.
Pseudo-halftone processing enables an image-forming apparatus having a small number of displayable gradations per pixel to handle one-pixel data which are multivalued data and to form an image such as a full-color image therefrom. The pseudo-halftone processing can generally be divided into AM screening and FM screening.
Typically, AM screening is a dither method of expressing a halftone image by forming dots that are aligned at a certain pitch and changing the sizes of the dots, while FM screening is an error diffusion method of expressing a halftone image by changing the density of the dots.
Generally, AM screening or FM screening is selected and designed as appropriate for improvement in the graininess of a halftone image. AM screening has been used in electrophotographic methods for a long time. However, AM screening has often resulted in generation of output moire and a noticeable hexagonal rosette pattern in highlights and halftone portions, and noticeable density jump even at an area rate of approximately 50%, because dots are formed in the above-described manner. To solve these problems regarding image quality, use of FM screening was proposed (JP-A Nos. 2003-189103 and 2004-1260).
Recently, there has also been a need for power saving and improvement in image quality in electrophotography. To save power in electrophotographic methods and specifically, to reduce the energy consumption needed during operation of the device, there is a need for low temperature image fixing. For reduction of the energy consumption during stand-by periods, apparatuses are often used that have a function of reducing the power supply to the fixing unit and thus cooling the heater such as a heating roll to a temperature lower than that during fixing (hereinafter, referred to “power-saving function during stand-by period”) when the stand-by mode where no image is formed is continued for a certain period.
To ensure efficiency of operation as well as power saving, it is preferable to use a fixing unit having a smaller heat capacity in apparatuses having such a function. This is because, when an apparatus containing a heater cooled to a temperature lower than that suitable for fixing by a decrease in energy supply to the heater in the fixing unit is reactivated to the state for image forming, it is desirable, from the viewpoint of efficiency, to heat the heater to a temperature suitable for fixing as soon as possible by increasing power supply.
However, when an image-forming apparatus containing such a fixing unit is reactivated, from the stand-by mode where the heater of the fixing unit is cooled to a temperature lower than the set fixing temperature of the heater to an image-forming mode, a great amount of electric energy is applied to the heater all at once to instantaneously heat the heater to the set fixing temperature, resulting in a phenomenon that the heater temporarily overheats to a temperature higher than the set fixing temperature (overshoot). Such an overshoot immediately after resumption of image forming is called “initial overshoot”. When paper is fed into the fixing unit for image formation during overshoot, the paper in the fixing unit absorbs the heat of the heater and chills the fixing unit to a temperature lower than the temperature elevated by the overshoot.
When images are formed successively, the decrease in the temperature of fixing unit by paper supply, and the increase thereof by heating when the fixing unit has cooled to a temperature lower than a predetermined temperature, occur periodically, causing periodic overshoot (hereinafter, referred to as “periodic overshoot”).
Such an overshoot may cause offsetting when it is significant or when it is not significant but a particular toner is used in the apparatus. In particular, when the offsetting occurs at high temperature, the offsetting becomes more obvious in areas where a halftone image is formed than in areas where a solid image is formed (hereinafter, offsetting of the halftone image is referred to as “halftone offsetting”).
On the other hand, when an image is formed with a conventional toner having superior low-temperature fixing property and a narrow melting temperature range, increase in the temperature of the fixing unit during fixing tends to cause halftone offsetting more frequently.
Thus, there is a need for an image-forming method that allows low-temperature fixing and suppresses halftone offsetting even at a high fixing temperature and an image-forming apparatus using the same.